Multi-species models of Antarctic krill predators : do competitive effects influence estimates of pre-exploitation whale abundance and recovery?

• Main Author: Mkango, Sara
• Other Authors: Butterworth, Doug S
• Format: Dissertation
• Language: English
• Published: University of Cape Town 2014
• Subjects: Mathematics and Applied Mathematics
• Online Access: http://hdl.handle.net/11427/4911

Includes abstract. Includes bibliographical references (leaves 92-111). === Many species of baleen whales and seals in the Southern Hemisphere were subject to intensive overexploitation by commercial harvesting in the last two centuries, and many populations were reduced to very low levels. Krill is the dominant prey item of these species. Harvesting (to near extinction) of the large baleen whales (blue, humpback and fin whales) from the start of the 20th century led to a likely increase in the availability of krill to other krill predators such as the Antarctic minke whales and crabeater seals. This phenomenon is referred to as the “krill surplus” hypothesis and has been a central hypothesis of Antarctic ecosystem studies. This thesis aims to better understand species interactions in the Antarctic through developing and extending multispecies models of the system. The study considered only Region A (IWC Management Areas II, III and IV, 60°W to 130°E) because the numbers of baleen whales harvested in Atlantic/Indian Oceans were far greater than in other Oceans, so that the impacts on the dynamics of these species are likely greater. The simple models of competition between blue and fin whales developed give qualitatively similar results to the Mori-Butterworth Antarctic ecosystem model of an initial number of fin whales before exploitation began that is much lower than single species models suggest. However, there are important features of blue and fin whale CPUE data off Durban over the middle decades of the last century that are not reflected by the model results, and a number of possible reasons for this are advanced. In particular, the introduction of competition in the models predicts a steady fin whale population until 1950, but cannot reproduce the feature in the CPUE data of an increase from the 1920’s to 1950’s. The study then extends the Mori-Butterworth Antarctic ecosystem model by adding squid, which has fast dynamics compared to whales and seals. The model estimates population trends in terms of numbers or biomass. This study indicates that results are particularly sensitive to the density dependence assumed for natural mortality and/or birth rate. The results highlight that the squid biomass trajectory is relatively insensitive to initial squid abundance but depends strongly on the density dependence assumed for squid. Generally, the estimated historical trajectories suggest that the inclusion of squid in the model hardly impacts the maxima reached by other species that benefited from the krill surplus. The model predicts that squid started to increase at about the same time (1920) that the reduction of large baleen whales (blue, humpback and fin whales) commenced under heavy harvesting. This suggests that species with fast dynamics such as squid were possibly the first to benefit from krill surplus, even before minke whales and crabeater seals, which started to increase only about a decade later. The study provides a potential framework for understanding the interplay between species with slow and fast dynamics.